Amyloid is a specific protein having a fibrous structure. Amyloid exhibits weak acidophilic characteristics in hematoxylin-eosin staining, and assumes homogeneous and amorphous. Amyloid stains orange-red with alkaline Congo red staining, and shows green birefringence under a polarizing microscope. As observed under an electron microscope, amyloid is composed of non-branching fibrils having a width of 7 to 15 nm. Although amyloid seems to have a single morphology, amyloid is conceived to be composed of at least 20 types of proteins. Such proteins, in a monomeric state, do not exhibit toxicity, but cause organ dysfunction when aggregated. A common feature of aggregates of these proteins resides in that they are rich in β-sheet structure and are hard to dissolve.
Amyloidosis is a group of diseases in which dysfunction is caused by extracellular deposition or accumulation of amyloid fibrils in various organs in the whole body. As described below, amyloidosis is classified into systemic and localized forms according to the new classification by the Specified Disease Research Group of the Japanese Ministry of Health, labor and Welfare.
I. Systemic Amyloidosis
1. Immunocytic Amyloidosis
Deposition of immunoglobulin-derived (X- or K-light-chain-derived, or heavy-chain-derived) amyloid in organs of the whole body.
2. Reactive AA Amyloidosis (Secondary Amyloidosis)
Secondary to chronic inflammatory diseases (e.g., rheumatoid arthritis, tuberculosis, leprosy, and bronchiectasis), and deposition of amyloid derived from serum amyloid A (SAA), which is an acute-phase protein.
3. Familial Amyloidosis (Hereditary Amyloidosis)
Familial amyloid polyneuropathy (classified into types I to IV) causes specific sensory disorder or dyskinetic neuropathy, or autonomic neuropathy (variant transthyretin). Other examples of familial amyloidosis include familial Mediterranean fever and Muckle-Wells syndrome.
4. Dialysis Amyloidosis
Some long-term dialysis patients may exhibit β2-microglobulin-derived amyloidosis.
5. Senile Amyloidosis
Accumulation of wild-type transthyretin in the heart, and pulmonary or gastrointestinal vascular walls.
II. Localized Amyloidosis
1. Cerebral Amyloidosis
Alzheimer's disease, Down syndrome, cerebrovascular amyloidosis, hereditary cerebral amyloid angiopathy, British familial dementia, and Creutzfeldt-Jakob disease.
2. Endocrine Amyloidosis
Amyloidosis associated with medullary thyroid cancer, type-II diabetes/insulinoma, and localized atrial amyloidosis.
3. Cutaneous Amyloidosis
4. Localized Nodular Amyloidosis
Since amyloid can accumulate in any organ of the body, systemic amyloidosis causes a variety of symptoms. In an early stage, systemic amyloidosis causes non-specific initial symptoms, including general malaise, weight loss, edema, and anemia. Known symptoms during the course of systemic amyloidosis include congestive heart failure, nephrotic syndrome, malabsorption syndrome, peripheral neuropathy, orthostatic hypotension, carpal tunnel syndrome, and enlarged liver. In a clinical examination of systemic amyloidosis, amyloid-constituent proteins are detected through a hematological and serological assay. Meanwhile, 99mTc-pyrophosphate scintigraphy is effective, but not specific for detection of cardiac amyloid, since 99mTc-pyrophosphate may also accumulate in an ischemic site.
Established diagnosis of amyloidosis requires collection, through biopsy, of a tissue from an organ suspected of having amyloid deposition, followed by confirming amyloid deposition. After a pathological examination of the tissue collected by biopsy, an amyloid precursor protein is specified through combination of an immunohistological test, a serological test, and a genetic test, leading to established diagnosis. Diagnosis of amyloidosis requires determination of a site where a large amount of amyloid is accumulated, and biopsy for collecting tissue from the site. Therefore, such a diagnosis may require a highly invasive test at a certain biopsy site.
Now will be described reactive AA amyloidosis, which is a typical example of systemic amyloidosis. Reactive AA amyloidosis is a disease caused by deposition of amyloid derived from serum amyloid A (SAA) (i.e., an acute-phase reactive protein), and is secondary to a chronic inflammatory disease. For example, rheumatoid arthritis, whose prevalence is 0.3 to 0.8% of the population in Japan, is a primary disease of AA amyloidosis, and complication occurs in about 10% of AA amyloidosis cases. This amyloidosis is a disease (complication) with very poor prognosis, in which various organ dysfunctions are caused by extracellular deposition of amyloid, and the 50% survival time of patients with this disease is two to four years. In general, diagnosis of the disease is limited to only a method for confirming amyloid accumulation through biopsy. Although this method targets the kidney or the gastrointestinal tract where AA amyloid is accumulated, the method is invasive, and is particularly difficult to perform in the kidney. Recently, imaging of peripheral amyloid accumulation has been successfully performed by use of SAP (serum amyloid P component) labeled with 123I (Non-Patent Document 1). However, SAP, which is a glycoprotein having a molecular weight of 250 kDa, cannot pass through the blood-brain barrier (BBB), and thus imaging of cerebral amyloid accumulation fails to be performed. As has been reported, SAP is accumulated specifically in the liver, the spleen, the kidney, the adrenal glands, the bone marrow, or joints, but is not accumulated in the heart.
Alzheimer's disease is a type of localized amyloidosis, and has become a serious social issue with aging of the population. In Japan, the number of dementia patients has been rapidly increasing in accordance with aging of the population, and thus treatment and care of dementia patients are imminent issues which must be rapidly solved from the viewpoint of health economics. The number of dementia patients is estimated to reach three million in 2050, and patients with Alzheimer's disease are expected to account for the majority of the dementia patients. In the United States, the number of Alzheimer's disease patients is currently four million, whereas in Japan, the number is estimated to be one million. Alzheimer's disease is a poor-prognosis disease which is associated with continuous progression and results in certain death; i.e., a half of Alzheimer's disease patients die within three to eight years after the onset of the disease.
Diagnostic imaging is an important examination for differentiating Alzheimer's disease from another amyloid disease. However, no evidence is obtained through contrast between autopsy and imaging. Attempts have been made to diagnose Alzheimer's disease in its early stage by means of CT or MRI for detecting cerebral atrophy, or PET or SPECT for measuring change in glucose metabolism or cerebral blood flow, so as to provide typical findings of Alzheimer's disease. However, such findings are not necessarily obtained from some Alzheimer's disease patients.
Established diagnosis of Alzheimer's disease requires pathological diagnosis, which is generally based on distribution of senile plaques, or Alzheimer neurofibrillary tangle; for example, according to CERAD (Consortium to Establish a Registry for Alzheimer's Disease), or the staging by Braak, et al. Of these, in CERAD pathological diagnostic criteria, the number of typical senile plaques stained with silver impregnation is semi-quantified, followed by classification into “sparse” (2/mm2), “moderate” (6/mm2), and “frequent” (35/mm2) at the neocortex with the most severe Alzheimer's lesion; and the level of Alzheimer's disease is evaluated as being “definite,” “probable,” or “normal” through comparison with the number of senile plaques normalized according to stratification of age.
As described above, established diagnosis of Alzheimer's disease requires data on distribution of senile plaques in the brain, or neurofibrillary tangle. Particularly, senile plaques are considered to have been accumulated for several decades before the onset of Alzheimer's disease, and detection of this amyloid accumulation meets the purpose of early diagnosis of Alzheimer's disease. In recent years, attempts have been made to capture such senile plaques in the form of an image. A nuclear medicine technique employing nuclear magnetic resonance or a radioisotope is considered a technique for in vivo imaging of cerebral senile plaques. Particularly, attempts have been made to directly image senile plaques through a nuclear magnetic resonance technique in APP-overexpressed transgenic mice (Tg mice). However, such a technique requires a high magnetic field of 7 T or higher, and provides low contrast with respect to senile plaques.
Furthermore, attempts have been made to develop a drug which binds specifically to amyloid for imaging thereof, for the purpose of enhancing sensitivity by increasing contrast between the normal tissue and amyloid. Recently, Higashi, et al. have developed a drug which passes through the blood-brain barrier (BBB) and binds to amyloid (Patent Document 1). However, clinical application thereof requires a ligand exhibiting higher activity.
Many attempts have been made to visualize amyloid aggregates by binding a radionuclide to a ligand which binds to the amyloid aggregates, and reconstituting an image by means of a γ-ray-detecting apparatus. Imaging of senile plaques in the brain of an Alzheimer's disease (AD) patient was first successfully performed in the world by use of [18F]-FDDNP, which is a radioactive drug developed by a group of UCLA. This drug realizes imaging of both senile plaques and neurofibrils (Patent Document 2). In addition, a significant difference was observed in retention time of the drug in the brain between AD patients and control subjects, and the retention time was found to be correlated with cognitive function. However, as shown in an image obtained in a late stage after administration of the drug, only a small difference (about 10 to about 30%) is found in radioactivity retention in the brain of an AD patient between the cerebral cortex where many senile plaques are present and the pons where few senile plaques are present; i.e., a large background is provided.
A new amyloid imaging agent having a thioflavin structure has been developed in the University of Pittsburgh, and is named Pittsburgh Compound-B (11C-PIB) (Patent Documents 3 and 4). As has been reported, when 11C-PIB is administered to a mild AD patient, considerable retention of the agent is observed in an amyloid-accumulated cortex region, as compared with the control, and thus the agent realizes clear distinction between an Alzheimer's disease patient and a normal subject. However, a difference in retention of 11C-PIB between AD patient groups is at most about two-fold. Therefore, demand has arisen for a diagnostic drug which can achieve a higher contrast for examining the degree of amyloid accumulation in more detail. Such a drug is essential for determining whether or not a patient with a symptom which is more difficult to diagnose (e.g., mild cognitive impairment (MCI)) will develop Alzheimer's disease. As has also been reported, when 11C-PIB is administered to mice in which APP is overexpressed so as to generate amyloid aggregates (PS1/APP mice), and then the mice are tested by means of an animal PET scanner, no difference is observed in retention time of 11C-PIB between PS1/APP mice and normal mice; i.e., 11C-PIB exhibits considerably low affinity to amyloid aggregates of PS1/APP mice, and thus imaging of amyloid accumulation fails to be attained in vivo animal experiments by use of 11C-PIB. Furthermore, 11C, which is a labeling nuclide of 11C-PIB, has a half-life of 20 minutes. In general, such a positron-emitting nuclide employed for imaging has a very short half-life. In view that a compound is labeled with a radionuclide produced by means of a cyclotron, the compound must be capable of being labeled within a short period of time, and the cyclotron must be placed in the vicinity a PET apparatus. In addition, the thus-labeled compound must be under strict quality control, since the compound is administered in vivo. Therefore, in Japan, there are only a few facilities which can perform labeling with a positron-emitting nuclide, as well as imaging of amyloid in vivo.
Commercially suppliable and industrially useful means is a drug product labeled with a nuclide which has a longer half-life (about 6 to about 72 hours) and emits γ-rays. Examples of the nuclide which meets this requirement include 123I and 99mTc. The University of Pennsylvania has reported that 123I-IMPY, which has been studied for the purpose of 123I labeling, exhibits high affinity to amyloid accumulated in the brain of AD patients or Tg mice (Patent Document 5). The amyloid-binding probe having a thioflavin skeleton labeled with a radioisotope has an N-alkylamine structure. In general, an N-alkylamine structure undergoes in vivo metabolism. Therefore, an N-dealkylated radioactive ligand metabolite having no amyloid-binding property and exhibiting lipid solubility passes through the blood-brain barrier, and enters the brain, whereby the metabolite is accumulated therein regardless of amyloid (Non-Patent Document 2). Therefore, keen demand has arisen for development of a derivative, a metabolite of which is not detected in the brain.
Conceivably, a substance which inhibits aggregation and/or deposition of amyloid (including amyloid protein and amyloid-like protein) is effective for prevention or treatment of amyloidosis. Prevention or treatment of cerebral amyloidosis (e.g., Alzheimer's disease) requires a diagnostic drug or therapeutic drug which can pass through the blood-brain barrier.
Patent Document 1: WO 2005/042461 pamphlet
Patent Document 2: Japanese Kohyo Patent Publication No.
Patent Document 3: WO 2004/083195 pamphlet
Patent Document 4: Japanese Kohyo Patent Publication No.
Patent Document 5: Japanese Kohyo Patent Publication No.
Non-Patent Document 1: Hawkins P. N., Lavender J. P., Pepys M. B., N. Engl. J. Med. 1990 Aug. 23; 323(8): 508-13.
Non-Patent Document 2: Kung M. P., Hou C., Zhuang Z. P., Cross A. J., Maier D. L., Kung H. F., Eur. J. Nucl. Med. Mol. Imaging. 2004 August; 31(8): 1136-45. Epub 2004 March 9.